With the United States in the clutches of a heat wave that is spurring deadly wildfires across the Southwest, it might feel like the sun is punishingly close right now. Actually the exact opposite is true: Earth will be farther from the sun on Friday, July 5, than on any other day this year.

That's because the orbits of all the planets in our solar system—including Earth's—are elliptical, as opposed to circular. The phenomenon was first explained in mathematical detail by the 17th-century German astronomer Johannes Kepler.

Earth's elliptical orbit means there will be a point each year when the planet is closest to the sun, called perihelion, and a point when it is farthest away, known as aphelion.

On Friday at 10:46 a.m. ET our planet will be at aphelion—94,508,959 miles (152,097,426 kilometers) from the sun. This year's perihelion was on January 1 at 11:39 p.m. ET, when Earth was 91,402,559 miles (147,098,161 kilometers) from the sun.

The timing and distances change slightly from one year to the next due in part to small variations in the Earth's orbital elements, said Mark Hammergren, an astronomer at the Adler Planetarium in Chicago, Illinois.

"The difference in dates is due more to the fact that the time it takes the Earth to complete one full orbit of the Sun"—one year—"is not an integer number of days, so the exact times of perihelion and aphelion vary from one year to the next," he added.

On average, Earth is about three million miles—or about 3 percent—farther from the sun at aphelion than at perihelion. As a result, the sun in the sky will look about 3 percent smaller, but you may not notice the difference without a telescope.

"This is not like the moon, where it can vary as much as 12 percent ... There's no 'supersun,'" said Joe Rao, a meteorologist and a guest lecturer at New York's Hayden Planetarium.

So if Earth is at its maximum distance from the sun, why are those of us in the Northern Hemisphere sweltering in the heat of summer?

Because it's the tilt of the Earth, not our distance from the sun, that determines the seasons.

Earth's north-south axis is tilted by about 23.4 degrees, so during its orbit the Poles point in different directions from the sun.

By coincidence, Earth reaches aphelion when the North Pole is tilted more toward the sun than the South Pole is.

"Because the Earth has a tilt, it means that in the summer months [the Northern Hemisphere] receives a longer duration of sunshine—so the day is longer and the night is shorter—but also the sunlight hits the ground more vertically," Hammergren said.

In fact, the Northern Hemisphere receives nearly twice as much sunlight during this time of year than in the winter. Because the sun takes a high angular path across the sky in the summer, it is in the sky for about 15 hours during this time of the year.

"In the wintertime the sun takes a low path ... and is in the sky for just nine hours," Rao said. "You lose six hours of daylight."

Because of the way the Earth absorbs the sun's heat, the worst of summer still lies ahead. "The hottest time of the year doesn't coincide with the solstice, and it doesn't coincide with early July," Rao said.

"There's actually an atmospheric lag—it takes the better part of four to five weeks before we reach the hottest point of the year, which will come in late July or August."

Our planet's oceans take even longer to absorb heat than the atmosphere does, which explains why the hurricane season doesn't peak until September.

"The hurricane season reaches its peak on the 10th of September," Rao said. "That's almost three months past the summer solstice. That's because hurricanes feed on warm ocean water and the waters reach their temperature peak in the early to middle part of September."